I think one concern is that MS has never properly nurtured studios and helped them achieve more than that they could have. That's the reason none of their first-party studios could compete, so Microsoft instead of nurturing their studios to become great went on a buying spree and bought studios that were very good.
Chances are those good studios will continue making good games (e.g., Arkane, Bethesda, etc.), but based on past records, there is an equal chance that the quality will go down (e.g., could be because of MS's management style, focus on keeping a steady supply of games for Gamepass, etc.).
If a studio works with Sony, we know, based on past records, they will get better and better, e.g., SuckerPunch, Bend Studio, Bluepoint, HouseMarque, Insomniac, etc.
Bioware's acquisition of EA is a shining example of how a studio can easily go down the drain. MS's own Rare is another example.
But I agree with you -- at least for Bethesda studios -- there are more chances of them making a good game than a bad one. But I don't think all these studios will start producing bangers just because they are with MS now. I foresee a lot of hits and misses.